Issue error if any complex assignment operator is used with a struct type.

Issue #275.

tests_errors/struct_arith.ispc

@@ -0,0 +1,8 @@
+// Assignment operator "+=" is illegal with struct type
+
+struct Point { float x, y, z; };
+
+void foo() {
+    Point a = {1,2,3}, b = {4,5,6};
+    a += b;
+}